It is well known that light travels at different speeds in different materials. The change of speed results in refraction. The relative refractive index between two materials is given by the speed of an incident light ray divided by the speed of the refracted ray. If the relative refractive index is less than one, then light will be refracted towards the surface, e.g. light emerging from a glass block into air. At a particular angle of incidence "i", the refraction angle "r" becomes 90.degree. as the light runs along the block's surface. The critical angle "i" can be calculated, as sin i=relative refractive index. If "i" is made even larger, then all of the light is reflected back inside the glass block and none escapes from the block. This is called total internal reflection. Because refraction only occurs when light changes speed, it is perhaps not surprising that the incident radiation emerges slightly before being totally internally reflected, and hence a slight penetration (roughly one micron) of the interface, called "evanescent wave penetration" occurs. By interfering with (i.e. scattering and/or absorbing) the evanescent wave one may prevent (i.e. "frustrate") the total internal reflection phenomenon.
In a number of applications, it is desirable to controllably frustrate the phenomenon of total internal reflection. For example, if total internal reflection is occurring at an interface "I" as shown in FIG. 1A, the degree of reflection can be reduced by placing a dielectric material close to interface I such that dielectric D interacts with the evanescent wave penetrating beyond interface I, as shown in FIGS. 1B, 1C, and ID, in which the extent of frustration of total internal reflection is gradually increased, culminating in complete frustration (FIG. 1D).
It is desirable that dielectric D be an elastomeric material. Inevitably, at least some foreign particles "P" (FIG. 2A) are trapped between dielectric D and interface I; and/or, the opposing surfaces of dielectric D and interface I have at least some dimensional imperfections "X" (FIG. 2B) which prevent attainment of a high degree of surface flatness over substantial opposing areas of both surfaces. Such foreign particles, or such surface imperfections, or both, can prevent attainment of "optical contact" between dielectric D and interface I. Optical contact brings dielectric D substantially closer than one micron to interface I, thereby scattering and/or absorbing the evanescent wave adjacent interface I, thus preventing the capability of interface I to totally internally reflect incident light rays. If dielectric D is formed of an elastomeric material, the aforementioned adverse effects of such foreign particles and/or surface imperfections are localized, thereby substantially eliminating their impact on attainment of the desired optical contact. More particularly, as seen in FIGS. 2C and 2D, the elastomeric nature of dielectric D allows dielectric D to closely conform itself around foreign particle P and around surface imperfection X, such that optical contact is attained between dielectric D and interface I except at points very close to foreign particle P and around surface imperfection X. Since such points typically comprise only a very small fraction of the opposing surface areas of dielectric D and interface 1, sufficiently substantial optical contact is attained to facilitate frustration of total internal reflection as described above.
However, if an elastomeric material makes optical contact with a surface, the elastomeric material tends to stick to that surface and it is difficult to separate the two. This is because elastomeric materials are sufficiently soft that the material can deform into intimate atomic contact with the atomic scale structure present at any surface; and, because the resultant Van der Waals bonds have sufficient adhesion that it is difficult to remove the material from the surface. These factors make it difficult to use an elastomeric material to frustrate the total internal reflection phenomenon; and, they make it especially difficult to use an elastomeric material to control or vary the degree of total internal reflection. It is desirable to control the degree of frustration of total internal reflection by varying an interfacial pressure applied between dielectric D and interface I; and, in general, it is desirable to achieve such control with the least possible amount of pressure. The aforementioned Van der Waals bonding can require negative pressures of order 10.sup.4 Pascals for release, which is desirably reduced. The present invention addresses the foregoing concerns.